The present invention is directed to the field of image processing as predicate to driving a printing machine, such as an image setter which images films or plates to printed on a flexopress (a flexographic printer), and to the use of halftone image screens to produce enhanced desktop publishing image data.
Halftoning is a computer graphics technique for displaying an image, with many gray levels, on a binary imaging device in which the gray levels are approximated by variable-sized black and white dots. The image presentation is achieved by changing dot percentage of area coverage (intensity) from region to region.
A gray code, in computer mathematics, is a binary code in which sequential numbers are represented by binary expressions, each sequential one of which differs from the preceding by one place, only. Gray level is the value associated with a pixel in a digital image, representing the brightness of the original scene in the vicinity of the point represented by the pixel. This translates to a direct relationship to dot size or percent area covered. Gray scale is an optical pattern in discrete steps between light and dark bearing on resolution.
In digital imaging, a desktop publishing computer performs gray scale manipulation on the data input into such computer from an image source such as a digital camera. Gray scale manipulation has become an image enhancement technique in which the appearance of a digital image is improved by applying a point operator to each pixel in the image, thereby adjusting its gray level.
This gray level adjustment has been performed by various methods. Previously, halftone imaging has been used to produce high resolution halftone images by iterative multi-level, multi-resolution error pyramidal convergence processes. See Peli, U.S. Pat. No. 5,109,282.
Halftone frequency modulated color separations have been produced from continuous-tone multicolored artefacts. In such a method, the dot patterns of the different tone levels are different for each individual color separation, but are connected with each other through criteria specifying an overlapping dot ratio in conjunction with a translation prohibition, thereby substantially suppressing moire, disturbing effects of misregister (e.g. color shifts) and graininess that is often seen in highlights and midtones of reproductions with stochastic distribution of halftone dots. See Kienlin, et al., U.S. Pat. No. 5,548,407.
Others have attempted to generate enhanced halftone imaging by using blue noise masks, or by dither matrixing techniques, or by incorporating special filters, or by dispersed dot screening, or by cluster dot screening. Many of these techniques utilize analog algorithms.
With the increased use of desktop publishing, digital computers have provided the ability to development digital halftoning techniques which employ digital image processing to produce halftone output images from a continuous tone input image. Color images can be created by combining gray scale values of a number of selected component colors. Typical component colors, selected for hard copy print operations, may be cyan, magenta, yellow and black (CMYK). In such a system, an individual component color, such as cyan is represented within the digital environment of a computer by a series of gray values ranging from 0 to 255. A CMYK image can be generated through the combination of four gray scale images, one for each component color (CMYK).
However, as the colors (CMYK) are used to make composite color results, gray scale enhancement for such a screening application is relevant to the individual gray scale values (0 to 255), and independent of the resultant color crated by a combination. It makes no difference if such screening is conducted in an application of black dots (patterns) on a white background, or a pattern of one, or a combination of more than one, of the component colors on a white background.
In digital halftone processing, each continuous tone gray value is converted into a binary halftone pattern. While many pattern styles are available, a dispersed dot pattern (FM screening) or a cluster dot pattern (AM screening) have been commonly utilized. Dispersed dot patterns are created by using a variety of error diffusion techniques (stochastic generators) which provide a randomized grain effect pattern. The size of the halftone dot, whose shape can vary from screen format to screen format, is fixed for a given screen.
Cluster dot patterns are generated by selecting a frequency or distance between halftone dots for the particular screen. Different gray values are achieved by varying the frequency of the dots.
A computerized imaging enhancement process will include some sort of pattern generator, the format for which will define a screen. Regardless of screen pattern selected, as pattern intensity is increased (percentage of area covered), image darkness an thereby resolution changes. With digital formats and digital implementation, and increase in gray value (number) results in an increased percent coverage.
These prior image enhancing methods have had various disadvantages. The analog methods have provided good quality tone continuity and change, but when processed in a digital environment, can introduce image discontinuities, and introduce processing delays encountered in translating between the analog and digital states. Iterative methods provide smooth transitions and constant repetitive gray scale values, but are slower computational processes. Filtering and masking can introduce approximations which in themselves can introduce imaging errors.
Both FM screening and AM screening have been implemented in a digital processing environment. However, FM screening (dispersed dot screening patterns) provides good imaging only at low gray values, but poor imaging (i.e., grainy effect) at higher gray values. AM screening (cluster dot screening patterns, which are usually round, or square or elliptical--on an exclusive basis) provides good imaging at high gray values, but irregular imaging at low gray values. Moreover, if one were to supplement the use of FM screening at low gray values with the use of AM screening at high gray values, the resultant image would have obvious transition point which would be objectionable to an observer. Patterns and the resolution obtained with each at various gray values.
What is desired is an improved digital imaging technique which provides enhanced digital imaging over the entire range of gray scale values, and which thereby minimizes or eliminates visible transition points.
What is also desired is an algorithm for implementing the digital technique which can reduce the computational requirements and the resultant processing times in relation to other imaging techniques.
What is further desired is an apparatus to provide such enhanced digital imaging and for the implementation of the algorithm of enhanced digital imaging process.